This invention relates to a variable displacement pump used as an oil pump arranged to supply a lubricant to an internal combustion engine, an automatic transmission, and so on.
A patent document 1 (a pamphlet of International Publication No. WO 2008/003169 A1) discloses a variable displacement pump using a vane pump. This variable displacement pump includes a cam ring which is swingably disposed within a housing; a rotor which is disposed radially inside the cam ring, and which is arranged to rotate as a unit with a drive shaft; a plurality of vanes which extend in radial directions, and whose tip end portions are slidably abutted on an inner circumference surface of the cam ring; a first spring and a second spring which are arranged to urge the cam ring in a direction where an eccentric amount of the cam ring is increased, and whose spring constants are different from each other, and a control hydraulic chamber which is defined between the housing and the cam ring. In this structure, the eccentric amount of the cam ring is controlled by urging forces of the first spring and the second spring, and a control hydraulic pressure introduced into the control hydraulic chamber.
Moreover, a patent document 2 (Japanese Patent No. 5620882) discloses a variable displacement pump includes a first spring arranged to urge a cam ring in a direction in which an eccentric amount of the cam ring is increased; a second spring which is disposed to confront the first spring, and which is arranged to urge the cam ring in a direction in which the eccentric amount of the cam ring is decreased; and a control valve which is provided in the pump housing, and which is arranged to control a control hydraulic pressure introduced into a control hydraulic chamber. In this structure, the position of the cam ring is controlled by the first spring, the second spring, and the control valve so as to stepwisely increase the hydraulic pressure.